1. Field of the Invention
The present invention relates to thin film transistors.
2. Description of the Related Art
As one type of field effect transistor, a thin film transistor in which a semiconductor layer which is provided over a substrate having an insulating surface is used for a channel formation region is known. For the semiconductor layer used in the thin film transistor, techniques using amorphous silicon, microcrystalline silicon, and polycrystalline silicon are disclosed (see Patent Documents 1 to 5). Thin film transistors have been used for, for example, liquid crystal television devices and have been put to practical use as switching transistors of pixels which form a display screen.    [Patent Document 1] Japanese Published Patent Application No. 2001-053283    [Patent Document 2] Japanese Published Patent Application No. 05-129608    [Patent Document 3] Japanese Published Patent Application No. 2005-049832    [Patent Document 4] Japanese Published Patent Application No. 07-131030    [Patent Document 5] Japanese Published Patent Application No. 2005-191546